This application claims priority benefit based on GB Provisional Application No. 9714907.4, filed Jul. 16, 1997.
The present invention relates to the production of aromatic carboxylic acids, and, more particularly, it relates to a method for recovering and recycling excess hydrogen from process vent streams in the production of highly pure terephthalic acid where the production of the pure terephthalic acid comprises catalytically hydrogenating impure terephthalic acid, which typically contains 4-carboxybenzaldehyde (4-CBA), color bodies and other impurities, in aqueous liquid phase solution at elevated temperature and pressure.
The impure terephthalic acid is dissolved in water in an inert, e.g., nitrogen, atmosphere at a temperature and pressure sufficiently high to provide a solution in liquid phase, and the resulting solution is subjected to hydrogenation in the presence of a Group VIII metal and in the presence of an excess of hydrogen of from 1 to 7 moles over the stoichiometric amount required for the principal reducible impurities, i.e., 4-CBA. The reaction solution is then separated from the catalyst, and pure terephthalic acid (PTA) is isolated from the solution by, typically, a series of crystallization steps in which the solution is cooled by releasing the pressure which, in turn, vaporizes water and dissolved inert gas from the solution and terephthalic acid crystals precipitate. U.S. Pat. No. 3,584,039 reports that vaporized water can be condensed and recycled if desired to the dissolution step. However, pressure reduction also causes excess hydrogen and inert gas to separate from the solution along with other volatile compounds formed by partial decomposition of terephthalic acid and its principal intermediates. Typical decomposition reactions include decarbonylation of 4-CBA to produce C6H5COOH+CO and decarbonylation of terephthalic acid to produce C6H5COOH+CO2. Carbon monoxide (CO) is a well known poison for hydrogenation catalysts. Depending on the activity level of the hydrogenation catalyst, the decarbonylation reactions (and others) can generate appreciable levels of gaseous impurities in the vented vapor stream, and these impurities, if recycled to the hydrogenation reaction, can be problematic. Consequently, excess hydrogen and other non-condensable components in the vent stream are usually passed through a vent scrubber of some kind and then released to the atmosphere.
The present invention provides for recovering and recycling excess hydrogen from process vent streams in the production of highly pure terephthalic acid where the production of the pure terephthalic acid comprises catalytically hydrogenating impure terephthalic acid, which typically contains 4-carboxybenzaldehyde (4-CBA), color bodies and other impurities, in aqueous liquid phase solution at elevated temperature and pressure. Commercial processes for producing pure terephthalic acid from an impure terephthalic acid having as impurities 4-carboxybenzaldehyde, color bodies and color forming precursors typically include the steps of:
(1) forming an aqueous solution containing the impure terephthalic acid in an inert atmosphere;
(2) treating the aqueous solution with hydrogen, or a prehumidified hydrogen-containing gas, at a concentration of from 1 to 7 moles above the stoichiometric amount required to reduce the 4-carboxybenzaldehyde present in solution to p-toluic acid in the presence of a Group VIII Noble metal catalyst at elevated temperature and pressure sufficient to maintain the solution in the liquid phase;
(3) separating the treated solution from the catalyst; and
(4) crystallizing terephthalic acid from the separated solution in one step, or in a series of graduated steps, by releasing the pressure on the treated solution whereby water and dissolved inert gas vaporize and a vent stream comprising water vapor, inert gas, unreacted excess hydrogen and volatile impurities is formed while non-volatile impurities and their reduction products remain dissolved in the resulting mother liquor. According to the present invention, hydrogen is recovered and recycled to the purification process by:
(a) cooling the vent stream to condense water vapor and condensable impurities;
(b) treating the vent stream to remove uncondensed volatile impurities and recover the hydrogen; and
(c) returning the hydrogen to the hydrogenation reaction.
The vent stream, exiting the first crystallizer, or exiting a series of graduated crystallizers, as the case may be, at a temperature in the range of from as low as 210xc2x0 C. to as high as 280xc2x0 C. and a pressure of around 2000 kPa absolute (abs.) to 6000 kPa abs., is first passed to a condenser, e.g., a feed preheater, wherein heat is transferred from the vent stream to a process feed or other stream thereby making use of available heat. Condensate and uncondensed vapors then pass to a condensate pot or other suitable means for collecting condensate. The vent stream, now at a temperature in the range of 236xc2x0 C. and a pressure of around 3200 kPa abs. and comprising steam, unreacted excess hydrogen, inert gas and generally non-condensable components selected from CO, CO2, CH4 and sulfur-containing compounds is let down in pressure to a pressure of around 2000 kPa abs. to insure that the vapors are superheated by about 15xc2x0 C. above their dew point prior to treatment. The vapors are then treated, i.e., purified, to selectively remove or chemically convert gaseous impurities which otherwise could be harmful if introduced to the hydrogenation reaction, particularly the Group VIII Noble metal catalyst. Although in practice it may not be possible to completely remove all gaseous impurities from the recovered hydrogen stream, the purified stream can be purged to maintain the level of gaseous impurities during operation within acceptable limits. The stream of recovered hydrogen is then compressed to a pressure in the range of 10,000 kPa abs., supplemented with fresh hydrogen, and returned to the hydrogenation reactor.